Why would you butter the inside?! You only need to put a little bit of butter on the outside. That's like people who when they make a normal sandwich, put both butter and mayonnaise on the inside! Pointless as you can only taste the mayo.

If I'm having mayo on a sarnie I still butter the bread, but I don't put mayo on both sides, the mayo just goes on the top of the sarnie. I think you can taste both though, the same as you can taste the butter and the brown sauce on a bacon sarnie.